PET Preform Injection Molding Practical Guide (Full Process & Troubleshooting)

2026-06-24 18:01:18

PET bottle preform injection molding is the core precision process in the production of plastic bottles in packaging fields such as beverages, food, cosmetics, and medicine. Standardized production processes, standardized operation procedures, and a complete quality control system are the core keys to ensuring the transparency, dimensional accuracy, structural toughness, and the later blow molding yield rate of the bottle preforms. This article is a common and practical technical guide for the industry, comprehensively covering the raw material processing, process control, production process, equipment maintenance, and common defect improvement plans for PET bottle preform injection molding, helping export production enterprises stabilize product quality, reduce production losses, and improve production efficiency. It is applicable to conventional industrial mass production and high-precision customized bottle preform production scenarios.

1. Raw material selection and pre-treatment process

The quality of raw materials and the pre-treatment effect directly determine the basic quality of the bottle preform products. Incorrect selection or inadequate pre-treatment can cause various appearance defects and performance problems during injection molding, resulting in batch quality abnormalities.

1.1 Raw material selection principles

New PET virgin particles for packaging bottle preforms are the preferred raw materials. Qualified raw materials have characteristics such as stable molecular viscosity, good melt flowability, high transparency, and excellent tensile properties, which are suitable for stable injection molding and later blow molding processing. It is strictly prohibited to mix in complex components and unstable viscosity recycled materials to avoid problems such as uneven melt filling, poor product consistency, and cracking during blow molding. High-end packaging products can use high transparency and low crystallization special PET raw materials to ensure uniform surface luster and transparent texture of the bottle preform, meeting the quality requirements of high-end export orders.

1.2 Dehumidification and drying pre-treatment

PET materials have extremely strong hygroscopicity and are prone to absorbing moisture from the air. Moisture-containing raw materials will undergo hydrolysis and degradation in the high-temperature injection molding environment, damaging the PET molecular structure, causing the surface of the bottle preform to become cloudy, internal defects, and a significant reduction in overall toughness, which seriously affects the qualification rate and performance of the finished products.

Before production, professional molecular sieve dehumidification and drying equipment must be used to uniformly remove the internal moisture of the raw materials at a constant temperature. The PET particles after drying must be well protected in a sealed environment and must not be exposed to the air for a long time to prevent secondary moisture absorption, ensuring that the raw materials for the injection molding process are completely in line with the injection molding production standards.

2. Key points of injection molding core process control

Stable process regulation is the core for stable mass production of PET bottle preforms. Reasonable temperature, pressure, and cooling logic can effectively avoid various production problems such as internal stress, surface defects, dimensional deviations, and deformation warping of the bottle preform.

2.1 Barrel temperature control

The injection barrel of the injection molding machine adopts a segmented gradient temperature rise mode, gradually increasing the temperature from the feed section to the nozzle section, ensuring that the PET raw materials are fully and uniformly melted. Uniformly melted melt can effectively avoid local overheating carbonization and molecular deterioration of the raw materials, eliminating appearance defects such as yellowing, black spots, and impurities on the bottle preform surface. During production, long-term over-temperature operation must be avoided to prevent the degradation of PET material properties and affect the structural strength and transparency of the bottle preform.

2.2 Injection and holding pressure process control

During injection molding, a variable pressure and variable speed molding logic is adopted, with rapid filling in the early stage to ensure the cavity is filled, and low-speed stable pressure in the later stage to eliminate internal stress of the product. Reasonable holding pressure process can effectively compensate for the volume shrinkage of the raw materials during the cooling process, completely solving problems such as surface scarring, depression, and unevenness of the bottle preform. The plasticization process maintains a low-speed and low-shear operation of the screw to avoid performance changes of the raw materials due to overheating, ensuring the stability of the melt state.

2.3 Mold and cooling control

The uniformity of mold temperature directly determines the surface smoothness and dimensional stability of the bottle preform. A constant-temperature and low-temperature mold environment is more conducive to the rapid and uniform shaping of the bottle preform. The scientific arrangement of the mold cooling waterway ensures balanced heat dissipation in each cavity, avoiding uneven cooling speeds in different parts of the product, and reducing problems such as warping, deformation, and eccentricity of the bottle preform from the root cause. Optimize the cooling duration reasonably to ensure the bottle molds are fully shaped and the quality is stable, thereby optimizing the overall production cycle and enhancing the production efficiency.

3. Standardized Injection Molding Production Process

A unified and standardized production operation process can effectively avoid human operational errors and ensure the uniform and stable quality of batch bottle molds, meeting the production requirements for large-scale export orders.

First, complete the raw material loading and constant temperature dehumidification drying operations, test the moisture content and plasticization state of the raw materials, and only then proceed with machine production after meeting the standards. Subsequently, preheat and debug the injection molding equipment and molds, check the operation status of core components such as nozzles, hot runners, and exhaust structures, calibrate the process parameters, and ensure the stable equipment condition.

After equipment debugging is completed, the screw uniformly melts the PET raw materials and uniformly and under high pressure injects the stable melt into the precise mold cavity. Through the stabilization pressure holding, balanced cooling process, the defects caused by raw material shrinkage are compensated, ensuring the complete and shaped bottle mold structure. After cooling and shaping, the mold is smoothly opened and the product is gently ejected to avoid scratches on the bottle mold surface and structural deformation.

Finally, conduct full inspection of the finished products and dust-free packaging operations. Screen for appearance, structure, and size defects one by one. Qualified products are sealed and stored in a dust-free environment to ensure product cleanliness and integrity, meeting the quality standards for export.

4. Daily Maintenance of Molds and Equipment

The standardized daily maintenance of molds and injection molding equipment is the basis for extending equipment lifespan, reducing failure probability, and ensuring long-term stable production. It can effectively reduce downtime, maintenance costs, and defective product output.

The bottle mold uses high-polished, corrosion-resistant high-quality mold steel, combined with optimized hot runner and exhaust structure, and the smooth exhaust system can effectively avoid defects such as product scorching, bubbles, and incomplete filling. Regularly clean the mold cavity and exhaust groove to keep the mold surface smooth and flat; regularly check the smoothness of the cooling waterway to prevent blockages that affect the heat dissipation effect; regularly lubricate and maintain the moving parts of the mold to ensure smooth and precise opening and closing of the mold.

Injection molding equipment needs to regularly clean the carbon deposits and residual impurities in the screw and barrel to prevent the deterioration of materials from causing black spots and impurity defects in the bottle molds. Regularly calibrate the equipment temperature and pressure sensing system to ensure precise and stable process parameters, avoiding parameter deviations that cause batch quality fluctuations and ensuring the continuity and stability of production.

5. Common Product Defects and Improvement Schemes

During the PET bottle mold production process, due to factors such as process, equipment, operation, and environment, various quality defects may occur. Targeted optimization and adjustment of the process and equipment status can quickly restore stable production and improve the yield rate.

Bubbles and silver streaks on the bottle mold surface are mostly caused by excessive moisture content in the raw materials, poor equipment exhaust, and unreasonable injection speed. By strengthening raw material drying treatment, cleaning the equipment exhaust structure, and optimizing the injection speed ratio, these problems can be completely solved.

Shrinkage and depression in the product are mainly caused by insufficient holding pressure and insufficient cooling. By optimizing the holding pressure process and extending the cooling shaping time, ensuring the full filling and stable shaping of the melt, the shrinkage defects can be effectively eliminated.

Bottle mold warping and deformation are caused by uneven mold temperature and imbalance of internal stress in the product. Balancing the mold temperature distribution, optimizing the injection molding rhythm, and leaving sufficient stress release time can effectively improve the flatness of the product and stabilize the product structure size.

Bottle mold yellowing and black spot defects are mostly caused by excessive molding temperature, equipment carbonization aging, and impure raw materials. Appropriately reducing the molding temperature, thoroughly cleaning the equipment carbonization, and replacing with new pure raw materials can quickly restore the product appearance quality.

Edges of the product have burrs and spurs, mainly caused by excessive injection pressure, improper mold closure, and rapid filling speed. Optimizing the injection pressure and speed, checking and calibrating the mold closure accuracy can completely solve the burr problem and improve the regularity of the product.

Insufficient filling and incomplete product are related to insufficient injection power, poor material流动性， and blocked flow channels. Reasonably optimizing the mold temperature and pressure, cleaning the nozzle and flow channel to remove impurities and blockages, can ensure that the cavity is fully filled and produce complete and qualified bottle blanks.

6. Quality Control Standards for Finished Bottle Blanks

For PET bottle blanks for export, they must strictly follow the international common packaging quality and safety standards. A complete quality control system should be established in four major dimensions: appearance, size, performance, and hygiene.

In terms of appearance quality, qualified bottle blanks have a smooth and transparent surface, are clean and flawless, without bubbles, silver streaks, black spots, scratches, burrs, deformation, etc. The bottle mouth thread is regular and complete, which can perfectly adapt to subsequent filling, sealing, and assembly processes.

In terms of size accuracy, the overall specifications of the finished bottle blanks are uniform, the wall thickness is uniform, there are no obvious size deviations, and the batch products have high consistency, which is suitable for the stable processing of various blow molding equipment.

In terms of physical properties, the internal structure of the bottle blanks is uniform and dense, without stress cracking hazards. The blow molding extension performance is excellent, and the plastic bottles formed after molding have good pressure resistance and drop resistance, which can meet the requirements of long-distance transportation and daily packaging use.

In terms of hygiene and safety, the product is produced using food-grade safe raw materials, which are non-toxic, odorless, clean, and environmentally friendly. It complies with the safety standards for food and daily chemical packaging in the EU and other international regions, and fully meets the quality inspection requirements for export. Summary

PET bottle blanks injection molding is a refined and standardized precision manufacturing process. The stability of product quality relies on scientific raw material pre-treatment, precise process control, professional equipment maintenance, and full-process standard operation. By strictly following this practical guide for production control and process optimization, it is possible to effectively reduce product defect rates, stabilize batch production quality, optimize production cycles, and significantly enhance the production efficiency of the enterprise. This guide is applicable to the long-term process optimization and daily quality control of various PET bottle blank production lines, providing professional and reliable technical support for the mass production of high-quality bottle blanks for export.